I would suggest a gtx 560.
even though it is about 100$ more than a 550 if you are using your system for gaming at moderate/high settings in many new games a 550 is going to be a bit disappointing. That and its not very future proof. It'll go out of date quite quickly with the new generation of cards coming in the coming 3-6 months. When i say out of date i mean, wont be able to run at playable frame rates in soon to be released games.
However, the 560 is a good choice for gaming on a budget. Itll last longer in the gaming market. You get what you pay for. In my opinion the extra 100$ or so is worth it, but thats for games i play- rage bf3 crysis 2, all very graphically demanding. obviously games like WOW and starcraft II wont need as much power.
The ti in the gtx 560 ti is a bit of a gimmick. Yeah the titanium is lighter than steel, but who cares about gfx card weight when you have a desktop, but ti is the standard version of the card so unless you find a great deal on one or the other, it doesnt make a lot of difference.
So, id suggest a 560 model ti or not doesnt matter.
However i would also suggest taking a look at the hd 6950. For the same price you could get the same raw performance with twice the memory, which adds to the lifecycle, or futureproof-ness of the card.
If youre a hardcore nvidia fan thats cool, but pick a card that you like and can afford, dont base it on brand unless youre looking at warranties.
Btw check out these links, you may find them useful- http://www.hwcompare.com/9686/geforce-gtx-550-ti-vs-geforce-gtx-560-ti/http://www.hwcompare.com/8888/geforce-gtx-560-ti-vs-radeon-hd-6950-2gb/